Earth models contain data which characterise the properties of, and surfaces bounding, the geological features which form the earth's sub-surface, such as rock formations and faults. They are used to assist operations occurring in the earth's sub-surface, such as the drilling of an oil or gas well, or the development of a mine.
The domain of applicability of an earth model varies greatly and should be considered on a case by case basis. Some earth models are applicable only in the near vicinity of a particular oil or gas well, or mine. Others may be valid for an entire oil or gas field, or perhaps even over a region such as the North Sea or Gulf of Mexico.
The data in an earth model consists of measurements gathered during activities such as the seismic, logging or drilling operations of the oil and gas industry, and of interpretations made from these measurements. The data may be gathered above, on, or below the earth's surface. As the duration or number of sub-surface operations increases, more data is gathered. This data can be used to amend the relevant earth model, with the aim that it should characterise the geology and properties ever more accurately. Clearly, it will be conducive to the efficiency of these operations if amendments which enhance the accuracy of the earth model are made as quickly as possible, or appropriate.
The U.S. Pat. No. 6,766,254 summarizes the recent methods for updating an earth model off-line and in real-time. For the purpose of providing background information to the present invention, the content of the patent is fully incorporated herein.
Microseismic measurements can be characterized as a variant of seismics. In conventional seismic explorations a seismic source, such as airguns, vibrators or explosives, is activated and generate sufficient acoustic energy to penetrate the earth. Reflected or refracted parts of this energy are then recorded by seismic receivers such as hydrophones and geophones.
In passive seismic or microseismic monitoring there is no actively controlled and triggered source. The seismic energy is generated through so-called microseismic events caused by subterranean shifts and changes that at least partially give rise to acoustic waves which in turn can be recorded using suitable receivers. Relevant background information on instruments and methods can be found for example in the U.S. Pat. Nos. 6,856,575; 6,947,843; and 6,981,550 as well as the published international applications WO2004/070424; WO 2005/006020; and the published United States application US US 2005/0190649 A1.
A specific field within the area of passive seismic monitoring is the monitoring of hydraulic fracturing. Such a hydraulic fracturing operation includes pumping large amounts of fluid to induce cracks in the earth, thereby creating pathways via which the oil and gas may flow. After a crack is generated, sand or some other proppant material is commonly injected into the crack to prevent it from closing. The loose proppant filling provides a conductive pathway for the oil and gas to flow from the newly formed fracture into the wellbore.
In the field of microseismic monitoring the acoustic signals generated in the course of a fracturing operation are treated as microseismic events. However, use is made of the information available from the fracturing operation, such as timing and pressure. A well-known example of a set of microseismic data is the Carthage Cotton Valley data, evaluated for example by James T. Rutledge and W. Scott Phillips in: “Hydraulic stimulation of natural fractures as revealed by induced microearthquakes, Carthage Cotton Valley gas field, east Texas”, Geophysics Vol. 68, No 2 (March-April 2003), pp. 441-452. Data relevant for this invention are found in: Rutledge, J. T., Phillips, W. S. and Mayerhofer, M. J., “Faulting induced by forced fluid injection and fluid flow forced by faulting: an interpretation of the hydraulic fracture microseismicity, Carthage Cotton Valley Gas field, Texas”, Bulletin of the Seismological Society of America, Vol. 94, No. 5, pp. 1817-1830, October 2004.